Impeller Housing Disassembly: 2 Frozen Screws Options

[stephestrad]

I'm attempting my first partial engine disassembly and I have run into a stumbling block: Two of the screws in the front of the impeller housing are solidly frozen. Additionally, the slots of the heads are starting to strip out.

Removal attempts so far:
1. Soaked them overnight in WD-40.
2. Several raps with a hammer. (Normal hammer, struck the screwdriver handle, not the screw directly.)
3. Heated them with a propane torch (3 or 4 times, 30-60 seconds each).
- I tried with and without squirts of water post-heating.
- Before and after the heat, sprayed with PB Blaster.
- Was the heat a bad idea, given the magnesium housing?

The screws are still stubbornly in place. For what it's worth, the engine is horizontal, so the WD-40 and BP Blaster don't have the benefit of gravity to helping pull them into the threads.

Help!!! (IMG:style_emoticons/default/sad.gif)